The present invention relates generally to variable-directivity microphone devices, and more particularly to a variable-directivity microphone device in which at least three unidirective microphone units are combined in a specific arrangement, and the respective output signals of these microphone units are mixed with varied mixing ratios, whereby the directivity is widely varied, to perform zooming of the acoustic or sound image with ample sense of distance change as sensed by the listener, by use of variable resistors having a simple construction.
Heretofore, as a microphone device capable of varying the directivity, there has been a microphone device having an arrangement wherein two unidirective microphones are disposed opposing each other, and their outputs are mixed with varied mixing ratios. In this device, a final output signal is obtained by varying the mixing ratio to thus vary the directivity of the microphone device, from a state of non-directivity to bidirectivity, up to unidirectivity.
However, in this known microphone device, the range of variation of the directivity is narrow, and hence, there is a drawback in that it is impossible to obtain an acoustic image zooming effect with ample sense of the change in distance.
Accordingly, in order to overcome the above drawback, there was has proposed a "VARIABLE-DIRECTIVITY MICROPHONE DEVICE" in the U.S. patent application Ser. No. 142,845 now U.S. Pat. No. 4,308,425 which is assigned to the assignee of the present invention. In this previously proposed device, three primary sound-pressure gradient unidirective microphone units are arranged in a specific combination of positional relationship, and the respective outputs of the microphone units are mixed with varied mixing ratios. In the above device, the directivity can be varied within a wide range from a state of non-directivity to primary sound-pressure gradient unidirectivity and secondary sound-pressure gradient unidirectivity (referred to as secondary unidirectivity hereinafter). Furthermore, accompanied by the variation in the directivity, variation of the volume (zooming of the acoustic image) is possible while imparting an ample sense of distance change.
However, in this previously proposed microphone device, two variable resistors for varying the mixing quantity (the radio with which respective outputs of the microphones are mixed) which are respectively connected to two microphones to vary the above directivity and volume, two variable resistors for varying the mixed signal level of the outputs of three microphones, and a variable resistor for varying the frequency characteristic of a circuit for compensating the mixed signal frequency characteristic, that is, a total of five variable resistors are required. Hence, a variable resistor having a special construction comprising four ganged variable resistors in which the variable resistors respectively undergo different variation in resistance, must be used. The disadvantages of this approach are that the circuit cannot be constructed on a small scale and with low cost, and a large torque is required to drive the variable resistors.